I have nothing really concrete to add here but I want to explain a hypothesis I formed after having experienced Exalted Klaxxi and first learned of Y'shaarj.
First though, regarding, "why is there no Northrend or Silithus sha or whatever," I think is because powerful sorcery was not used on them the way it was for Y'shaarj. Or at least no creation of the physical manifestations of sha-like beings anyway...as someone mentioned, the sha were manifested into corporeality. Perhaps they exist more ephemerally in other places.
So anyway, I was seriously getting chills when I went into the underground chapel dedicated to Y'shaarj. We found out that all this time, the Mantid we were working for are essentially in alignment with the Twilight Hammer. Also this explained their relation as cousins to Nerubians as well as Qiraji (and you'll remember that the Mantid were eluded to much earlier when we first learned that Nerub and Qiraj were infact related; as they were cousin races to eachother they also had a third that was further east and south). The Mantid's huge hulking Kunchong beasts resemble the Qiraji's behemoths like Buru the Gorger don't they? And in Azjol'Nerub, huge scarab behemoths run all over the place and they have a very similar look to the Kunchongs and Gorgers.
Made me think that there is more evidence to support that all the Old Gods were actually split from Y'shaarj in the first place; as well as that the various insectoid races could potentially all be offshoots of one original insect race. Could be that as seven sha were split from Y'shaarj's breath, 6 Old Gods were created like cutting the heads off of hydras (and hydras are a favored pet of the Old Gods).
Therefore when people show the 5-pointed map creating a star with points in Silithus, Hyjal, Northrend, Tirisfal Glades, Stranglethorn Vale, and a 6th point inside the Maelstrom, then you go and add a point in Pandaria to the south and it creates a hexagram. This whole time we placed emphasis on the Maelstrom because it was at the center, but if Y'shaarj is the father of all Elder Gods then it could be that the Maelstrom and the original point of the Sundering was simply where one of his six spawn landed. Two went north, two went east, two went west; they essentially spread out, with them settling about equally apart from each other. Remember that there is a good chance that they are deep underground, but their presence is felt in these locations, as they appear to be able to affect those who dwell above them in these areas.
For some background on how I came to these conclusions, pour over these:
http://www.wowpedia.org/Old_Gods
http://www.wowhead.com/guide=1500
http://www.wowpedia.org/Old_Gods_speculation
http://www.wowpedia.org/File:OldGodMap.png?c=1
As for Wrathion, he is an extremely duplicitous liar, and he is very obviously playing Horde and Alliance against one another. The Burning Legion may just be another ruse that he is using to help trick ourselves into helping him...if in fact he is eluding to them. He's definitely trying to gain access to something, possibly more power from deeper knowledge.